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This is the first in a series of papers describing a theoretical process model for positive photoresist. This model, based upon a set of measurable parameters, can be used to calculate the response of photoresist to exposure and development in terms of image surface profiles. The model and its parameters are useful in many ways, from measuring quantitative differences between different resist materials to establishment of process sensitivities and optimization of the resist process within a manufacturing system. In this paper, the concepts of photoresist modeling are introduced by following the exposure and development of a photoresist film on silicon exposed by a uniform monochromatic light flux. This very simple example provides insight into the complex nature of the photoresist process for reflective substrates. The accompanying paper, "Characterization of Positive Photoresists," gives detail about measurement of the new photoresist parameters. It is supported by "In-Situ Measurement of Dielectric Thickness During Etching or Developing Processes" which discusses automated experimental techniques needed to establish photoresist development rates. These resist parameters provide a complete quantitative specification of the exposure and development properties of the resist. They also allow quantitative comparisons: lot to lot, material to material, and processing condition to processing condition. The fourth paper, "Modeling Projection Printing of Positive Photoresists," applies the process model to one technique of photoresist exposure. This paper contains the detailed mathematics of the model. The model is then used to calculate line-edge profiles For developed resist images.